Subject: Re: [PATCH] kasan: fix the missing underflow in memmove and memcpy with CONFIG_KASAN_GENERIC=y

From: Walter Wu <[email-masked]>
Date: Mon, 7 Oct 2019 11:22:56 +0800

On Fri, 2019-10-04 at 15:52 +0200, Dmitry Vyukov wrote:
> On Fri, Oct 4, 2019 at 2:05 PM Walter Wu <[email-masked]> wrote:
> >
> > On Fri, 2019-10-04 at 11:54 +0200, Dmitry Vyukov wrote:
> > > > > "out-of-bounds" is the _least_ frequent KASAN bug type. So saying
> > > > > "out-of-bounds" has downsides of both approaches and won't prevent
> > > > > duplicate reports by syzbot...
> > > > >
> > > > maybe i should add your comment into the comment in get_bug_type?
> > >
> > > Yes, that's exactly what I meant above:
> > >
> > > "I would change get_bug_type() to return "slab-out-of-bounds" (as the
> > > most common OOB) in such case (with a comment)."
> > >
> > >  ;)
> >
> >
> > The patchset help to produce KASAN report when size is negative size in
> > memory operation function. It is helpful for programmer to solve the
> > undefined behavior issue. Patch 1 based on Dmitry's suggestion and
> > review, patch 2 is a test in order to verify the patch 1.
> >
> > [1]https://bugzilla.kernel.org/show_bug.cgi?id=199341
> > [2]https://lore.kernel.org/linux-arm-kernel/[email-masked]/
> >
> > Walter Wu (2):
> > kasan: detect invalid size in memory operation function
> > kasan: add test for invalid size in memmove
> >
> > lib/test_kasan.c          | 18 ++++++++++++++++++
> > mm/kasan/common.c         | 13 ++++++++-----
> > mm/kasan/generic.c        |  5 +++++
> > mm/kasan/generic_report.c | 10 ++++++++++
> > mm/kasan/tags.c           |  5 +++++
> > mm/kasan/tags_report.c    | 10 ++++++++++
> > 6 files changed, 56 insertions(+), 5 deletions(-)
> >
> >
> >
> >
> > commit 0bc50c759a425fa0aafb7ef623aa1598b3542c67
> > Author: Walter Wu <[email-masked]>
> > Date:   Fri Oct 4 18:38:31 2019 +0800
> >
> >     kasan: detect invalid size in memory operation function
> >
> >     It is an undefined behavior to pass a negative value to
> > memset()/memcpy()/memmove()
> >     , so need to be detected by KASAN.
> >
> >     If size is negative value, then it will be larger than ULONG_MAX/2,
> >     so that we will qualify as out-of-bounds issue.
> >
> >     KASAN report:
> >
> >      BUG: KASAN: out-of-bounds in kmalloc_memmove_invalid_size+0x70/0xa0
> >      Read of size 18446744073709551608 at addr ffffff8069660904 by task
> > cat/72
> >
> >      CPU: 2 PID: 72 Comm: cat Not tainted
> > 5.4.0-rc1-next-20191004ajb-00001-gdb8af2f372b2-dirty #1
> >      Hardware name: linux,dummy-virt (DT)
> >      Call trace:
> >       dump_backtrace+0x0/0x288
> >       show_stack+0x14/0x20
> >       dump_stack+0x10c/0x164
> >       print_address_description.isra.9+0x68/0x378
> >       __kasan_report+0x164/0x1a0
> >       kasan_report+0xc/0x18
> >       check_memory_region+0x174/0x1d0
> >       memmove+0x34/0x88
> >       kmalloc_memmove_invalid_size+0x70/0xa0
> >
> >     [1] https://bugzilla.kernel.org/show_bug.cgi?id=199341
> >
> >     Signed-off-by: Walter Wu <[email-masked]>
> >     Reported -by: Dmitry Vyukov <[email-masked]>
> >     Suggested-by: Dmitry Vyukov <[email-masked]>
> >
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 6814d6d6a023..6ef0abd27f06 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -102,7 +102,8 @@ EXPORT_SYMBOL(__kasan_check_write);
> >  #undef memset
> >  void *memset(void *addr, int c, size_t len)
> >  {
> > -       check_memory_region((unsigned long)addr, len, true, _RET_IP_);
> > +       if (!check_memory_region((unsigned long)addr, len, true, _RET_IP_))
> > +               return NULL;
> >
> >         return __memset(addr, c, len);
> >  }
> > @@ -110,8 +111,9 @@ void *memset(void *addr, int c, size_t len)
> >  #undef memmove
> >  void *memmove(void *dest, const void *src, size_t len)
> >  {
> > -       check_memory_region((unsigned long)src, len, false, _RET_IP_);
> > -       check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> > +       if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
> > +       !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
> > +               return NULL;
> >
> >         return __memmove(dest, src, len);
> >  }
> > @@ -119,8 +121,9 @@ void *memmove(void *dest, const void *src, size_t
> > len)
> >  #undef memcpy
> >  void *memcpy(void *dest, const void *src, size_t len)
> >  {
> > -       check_memory_region((unsigned long)src, len, false, _RET_IP_);
> > -       check_memory_region((unsigned long)dest, len, true, _RET_IP_);
> > +       if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
> > +       !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
> > +               return NULL;
> >
> >         return __memcpy(dest, src, len);
> >  }
> > diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> > index 616f9dd82d12..02148a317d27 100644
> > --- a/mm/kasan/generic.c
> > +++ b/mm/kasan/generic.c
> > @@ -173,6 +173,11 @@ static __always_inline bool
> > check_memory_region_inline(unsigned long addr,
> >         if (unlikely(size == 0))
> >                 return true;
> >
> > +       if (unlikely((long)size < 0)) {
> > +               kasan_report(addr, size, write, ret_ip);
> > +               return false;
> > +       }
> > +
> >         if (unlikely((void *)addr <
> >                 kasan_shadow_to_mem((void *)KASAN_SHADOW_START))) {
> >                 kasan_report(addr, size, write, ret_ip);
> > diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
> > index 36c645939bc9..23951a453681 100644
> > --- a/mm/kasan/generic_report.c
> > +++ b/mm/kasan/generic_report.c
> > @@ -107,6 +107,16 @@ static const char *get_wild_bug_type(struct
> > kasan_access_info *info)
> >
> >  const char *get_bug_type(struct kasan_access_info *info)
> >  {
> > +       /*
> > +        * if access_size < 0, then it will be larger than ULONG_MAX/2,
> > +        * so that this can qualify as out-of-bounds.
> > +        * out-of-bounds is the _least_ frequent KASAN bug type. So saying
> > +        * out-of-bounds has downsides of both approaches and won't prevent
> > +        * duplicate reports by syzbot.
> > +        */
> > +       if ((long)info->access_size < 0)
> > +               return "out-of-bounds";
> > +
> >         if (addr_has_shadow(info->access_addr))
> >                 return get_shadow_bug_type(info);
> >         return get_wild_bug_type(info);
> > diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
> > index 0e987c9ca052..b829535a3ad7 100644
> > --- a/mm/kasan/tags.c
> > +++ b/mm/kasan/tags.c
> > @@ -86,6 +86,11 @@ bool check_memory_region(unsigned long addr, size_t
> > size, bool write,
> >         if (unlikely(size == 0))
> >                 return true;
> >
> > +       if (unlikely((long)size < 0)) {
> > +               kasan_report(addr, size, write, ret_ip);
> > +               return false;
> > +       }
> > +
> >         tag = get_tag((const void *)addr);
> >
> >         /*
> > diff --git a/mm/kasan/tags_report.c b/mm/kasan/tags_report.c
> > index 969ae08f59d7..19b9e364b397 100644
> > --- a/mm/kasan/tags_report.c
> > +++ b/mm/kasan/tags_report.c
> > @@ -36,6 +36,16 @@
> >
> >  const char *get_bug_type(struct kasan_access_info *info)
> >  {
> > +       /*
> > +        * if access_size < 0, then it will be larger than ULONG_MAX/2,
> > +        * so that this can qualify as out-of-bounds.
> > +        * out-of-bounds is the _least_ frequent KASAN bug type. So saying
> > +        * out-of-bounds has downsides of both approaches and won't prevent
> > +        * duplicate reports by syzbot.
> > +        */
> > +       if ((long)info->access_size < 0)
> > +               return "out-of-bounds";
> 
> 
> wait, no :)
> I meant we change it to heap-out-of-bounds and explain why we are
> saying this is a heap-out-of-bounds.
> The current comment effectively says we are doing non useful thing for
> no reason, it does not eliminate any of my questions as a reader of
> this code :)
> 
Ok, the current comment may not enough to be understood why we use OOB
to represent size<0 bug. We can modify it as below :)

If access_size < 0, then it has two reasons to be defined as
out-of-bounds.
1) Casting negative numbers to size_t would indeed turn up as a "large"
size_t and its value will be larger than ULONG_MAX/2, so that this can
qualify as out-of-bounds.
2) Don't generate new bug type in order to prevent duplicate reports by
some systems, e.g. syzbot."

> 
> 
> 
> > +
> >  #ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> >         struct kasan_alloc_meta *alloc_meta;
> >         struct kmem_cache *cache;
> >
> >
> >
> > commit fb5cf7bd16e939d1feef229af0211a8616c9ea03
> > Author: Walter Wu <[email-masked]>
> > Date:   Fri Oct 4 18:32:03 2019 +0800
> >
> >     kasan: add test for invalid size in memmove
> >
> >     Test size is negative vaule in memmove in order to verify
> >     if it correctly produce KASAN report.
> >
> >     Signed-off-by: Walter Wu <[email-masked]>
> >
> > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> > index 49cc4d570a40..06942cf585cc 100644
> > --- a/lib/test_kasan.c
> > +++ b/lib/test_kasan.c
> > @@ -283,6 +283,23 @@ static noinline void __init
> > kmalloc_oob_in_memset(void)
> >         kfree(ptr);
> >  }
> >
> > +static noinline void __init kmalloc_memmove_invalid_size(void)
> > +{
> > +       char *ptr;
> > +       size_t size = 64;
> > +
> > +       pr_info("invalid size in memmove\n");
> > +       ptr = kmalloc(size, GFP_KERNEL);
> > +       if (!ptr) {
> > +               pr_err("Allocation failed\n");
> > +               return;
> > +       }
> > +
> > +       memset((char *)ptr, 0, 64);
> > +       memmove((char *)ptr, (char *)ptr + 4, -2);
> > +       kfree(ptr);
> > +}
> > +
> >  static noinline void __init kmalloc_uaf(void)
> >  {
> >         char *ptr;
> > @@ -773,6 +790,7 @@ static int __init kmalloc_tests_init(void)
> >         kmalloc_oob_memset_4();
> >         kmalloc_oob_memset_8();
> >         kmalloc_oob_memset_16();
> > +       kmalloc_memmove_invalid_size();
> >         kmalloc_uaf();
> >         kmalloc_uaf_memset();
> >         kmalloc_uaf2();

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